This month''s WoodenBoat Magazine has an excellent article on measuring an existing boat. Chappelle''s ''Boat Building'' and Chappelle''s ''Yacht Design and Planning'' have the simpliest discussions on lines drawings.
Having measured a number of boats, my best advice is to take your time extablishing a base lines and leveling the boat. The rest is time consuming but not all that hard.
On the other hand, depending on the particular design and designer its probably a lot easier and perhaps less expensive to purchase a set of drawings.